Continuous Pulse Mode - Keithley 2520 User Manual

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Continuous Pulse Mode

Continuous pulse mode
In addition to LIV characterization, laser diodes typically require some type of wavelength
or spectrum measurement. In these cases, the 2520 can be used as a current pulse
generator, driving the laser diode with a current pulse train while a spectrometer or other
instrument makes measurements. As a pulse generator, the 2520 provides a fixed pulse
current level with control over pulse width and pulse off time to provide a desired duty
cycle. As with conventional, single purpose pulse generators, no measurements are made
when using the 2520 in the continuous pulse mode. Ensure that the 2520 is using the latest
firmware (B07/DSP B06 as of this writing) before using the continuous pulse mode.
Pulse test advantage
Testing in the pulse regime is done to minimize the heating of the diode junction, so duty
cycles are typically 1% or less. By minimizing the heating, testing can be performed on
the laser diode as soon as the lasing cavity is defined, at the wafer level for VCSELs and at
the bar level for edge-emitting laser diodes. This early testing permits pass/fail and other
grading decisions to be performed before any additional process or packaging costs are
added to the device.
Duty cycle
The 2520 supports a wide range of duty cycles. There are two different maximum duty
cycles based on the current source level:
For I ≥ 1.00A, Maximum Duty Cycle = 4%
For I < 1.00A, Maximum Duty Cycle = 99.6%
The duty cycle (DC) is controlled by adjusting the pulse width (PW) and pulse delay (PD)
values. The pulse delay is the pulse off time. This gives:
DC% =
where: DC = duty cycle (%)
For example, the maximum duty cycle (for I < 1 A):
DC% =
× 100
PW
PW+PD
PW = pulse width (s), 500ns to 5ms
PD = pulse delay (s), 20µs to 500ms
PW = 5ms
PD = 20µs
× 100 = 99.6%
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Model 2520 User's Manual